(1) Field of the Invention
The invention relates to a method for producing cores and moulds for the foundry industry, and a mould material mixture such as is used in the method.
(2) Description of Related Art
Casting moulds for producing metal components consist of parts called cores and moulds. The casting mould is essentially a negative representation of the casting to be produced, and cores are used to create cavities within the casting while the moulds reflect the external delineation. In this context, different cores and moulds are subject to different requirements. Moulds possess a relatively large surface area for dissipating gases that are formed by the action of the hot metal during casting. Cores usually have a very small surface area by which these gases can be dissipated. Therefore, if an excessive quantity of gas is generated, there is a danger that gas will escape from the core and into the liquid metal, resulting in casting defects there. Accordingly, the interior cavities are often reflected by cores that have been hardened using a cold box binders, that is to say a polyurethane-based binder, while the outer contour of the of the casting is represented by less expensive moulds, such as a basic sand mould, a mould that is bonded using a furan resin or a phenolic resin, or by a permanent mould.
Casting moulds consists of a fire-resistant material, for example quartz sand, the grains of which are bonded with a suitable bonding material after demoulding to lend adequate mechanical strength to the casting mould. Accordingly, casting moulds are produced using a fire-resistant primary moulding material that is reacted with a suitable binder. The mould material mixture obtained from the primary moulding material and the binder is preferably flowable so that it can be introduced into a suitable hollow mould and compacted therein. The binder ensures that the particles of the primary moulding material are bonded together firmly, so that the casting mould has the required mechanical stability.
Either organic or inorganic binders may be used in the production of casting moulds, and such binders may be cured by cold or hot method. In this context, methods that are carried out essentially at room temperature, without heating the mould material mixture, are called cold methods. Curing is usually effected by a chemical reaction, which may be initiated for example by passing a gas-phase catalyst through the mould material mixture to be cured, or by adding a liquid catalyst to the mould material mixture. In hot methods, the mould material mixture is heated to temperature that is high enough for example to drive out the solvent contained in the binder, or to initiate a chemical reaction in which the binder is cured by crosslinking.
At present, a wide variety of organic binders is used to produce casting moulds, including for example polyurethane, furan resin, or epoxy acrylate binders, with which the binder is cured by addition of a catalyst.
The selection of a suitable binder is determined by the shape and size of the casting item to be produced, the production conditions, and the material that is used for the casting. Thus for example, polyurethane binders are frequently used in the production of large numbers of small casting items, because they allow of rapid cycle times and thus also volume production.
Methods in which the mould material mixture is cured by heat or the subsequent addition of a catalyst have the advantage that processing of the mould material mixture is not subject to any time restrictions. The mould material mixture may be produced initially in relatively large quantities, which are then processed within a protracted period of time, usually several hours. The mould material mixture is not cured until after the moulding operation, though when curing does take place, the reaction should be a rapid as possible. The casting mould may be removed from the moulding tool immediately after curing so that short cycle times may be achieved. However, in order to ensure that the casting mould has good stability, curing of the mould material mixture in the casting mould must take place evenly. If the mould material mixture is to be cured by the subsequent addition of a catalyst, the gas-phase catalyst is passed through the casting mould after the moulding operation. To this end, the gas-phase catalyst is fed through the casting mould. The mould material mixture is cured directly upon contact with the catalyst, and may therefore be removed from the moulding tool very quickly. The larger the casting mould is, the more difficult it becomes to supply a sufficient quantity of catalyst to all sections of the casting mould to ensure that the mould material mixture will be cured. Gas exposure times become longer, and it is still possible for there to be sections in the casting mould that receive inadequate exposure to the gas-phase catalyst, or even none at all. Consequently, the quantity of catalyst increases significantly as the casting mould becomes larger.
Similar difficulties are encountered with hot curing methods. In this case, all sections of the casting mould must be heated to a sufficiently high temperature. As the casting mould increases in size, the times for which it must be heated to a specified temperature to enable curing become longer. Only then can it be ensured that the interior of the casting mould will have the requisite strength as well. Furthermore, as the size of the casting mould increases, the equipment that must be used for curing becomes very complex.
Consequently, when casting moulds are produced for large cast items, such as engine blocks for marine diesels or large machine parts such as rotor hubs for wind turbines, the binders uses are mostly of the no-bake type. In the no-bake method, the fire-resistant primary moulding material is initially covered with a catalyst. Then, the binder is added and by mixing is spread evenly onto grains of the fire-resistant mould material mixture that has previously been coated with the catalyst. The mould material mixture may then be shaped in the form of a mould. Since the binder and catalyst are both distributed evenly throughout the mould material mixture, curing takes place with a high degree of uniformity even for large moulds.
Since the catalyst is added to the mould material mixture before the moulding operation, the mould material mixture begins curing as soon as it has been produced. In order to achieve a processing time that is suitable for industrial application, one requirement is that the components of the mould material mixture must be adjusted to each other very precisely. This enables the reaction speed for a given quantity of binder and fire-resistant primary moulding material to be controlled by changing the type and quantity of the catalyst, or even by adding retarding components. The mould material mixture must also be processed under very closely controlled conditions, because the rate of curing is affected by the temperature of the mould material mixture, for example.
The classic no-bake binders are based on furan resins and phenolic resins. They are available commercially as two-component systems, in which one component is a reactive furan resin or phenolic resin and the other component comprises an acid that functions as the catalyst for curing the reactive resin component.
Furan and phenolic resins have very good dissociation properties during casting. The furan or phenolic resin is broken down by the heat of the molten metal, and the casting mould loses its stability. As a result, it is very easy to pour the cores out of the cavities after casting, after shaking the cast item if necessary.
The essential component of the reactive furan resins that represent the primary component of “furan no-bake binders” is furfuryl alcohol. With an acid catalyst, furfuryl alcohol is able to react with itself to form a polymer. In general, the furfuryl alcohol used to produce furan no-bake binders is not pure, other compounds are added to the furfuryl alcohol and are incorporated in the resin by polymerisation. Examples of such compounds are aldehydes such as formaldehyde or furfural, ketones such as acetone, phenols, urea, or also polyols such as sugar alcohols or ethylene glycol. Still other components can also be added to the resins to modify the properties of the resin, such as its elasticity. For example, melamine may be added to bind free formaldehyde.
Furan no-bake binders are usually obtained by a process in which precondensates containing furfuryl are first created for example from urea, formaldehyde and furfuryl alcohol in an acidic environment. The reaction conditions are selected such that only limited polymerisation of the furfuryl alcohol takes place. These precondensates are then diluted with furfuryl alcohol. Resols can also be used to produce furan no-bake binders. Resols are obtained by polymerising mixtures of phenol and formaldehyde. These resols are then diluted with furfuryl alcohol.
The second component of furan no-bake binders is an acid. This acid not only neutralised alkaline components that are contained in the fire-resistant primary moulding material, it also catalyses crosslinking of the reactive furan resin.
The acids most often used are aromatic sulfonic acids, and in some specific cases phosphoric acid or sulphuric acid as well. Phosphoric acid is used in a concentrated form, that is to say in concentrations greater than 75%. However, it is only suitable for the catalytic curing of furan resins that have a relatively high urea component. The nitrogen content in resins of this type is greater than 2.0% by weight. As a relatively strong acid, sulphuric acid can be added to weaker acids as a starter for curing furan resins. However, an odour characteristic of sulphur compounds is emitted during casting. There is also a danger that the casting material may absorb some of the sulphur, which would affect its properties of the material.
The compounds most commonly used as catalysts are sulfonic acids. Toluenesulfonic acid, xylenesulfonic acid, and benzenesulfonic acid are used particularly preferably because they are readily available and strongly acidic.
The choice of catalyst has a considerable effect on the properties of the binder. For example, the rate of curing can be adjusted by the quantity, and also by the strength of the acid. Larger quantities of acid, or stronger acids, both accelerate the curing rate. If too much catalyst is used, however, the furan resin becomes brittle during curing, and this in turn is detrimental to the strength of the casting mould. If too little catalyst is used, the resin is not cured completely, or curing takes a very long time, and this in turn impairs the strength of the casting mould.
When casting moulds are manufactured, most cores are made exclusively from new sand, while reprocessed sand is used for the moulds. Fire-resistant primary moulding materials that have been solidified using furan no-bake binders lend themselves very readily to reprocessing. Processing is carried out either mechanically, by mechanically abrading a shell formed from residual binder, or by heat treating the used sand. With mechanical processing or a combination of mechanical and thermal methods, recovery rates of close to 100% can be achieved.
The second large group of no-bake binders that are curable with acid catalysis are the phenolic resins, and the reactive resin component in these are resols, that is to say phenolic resins that have been manufactured with an excess of formaldehyde. Phenolic resins are markedly less reactive than furan resins, and strong sulfonic acids must be used as catalysts. Phenolic resins have a relatively high viscosity, which increases further if the resin is stored for a protracted period. This viscosity rises significantly, particularly at temperatures below 20° C., which means that the sand must be heated to enable the binder to be spread evenly over the surfaces of the sand grains. After the phenolic no-bake binder has been applied to the fire-resistant primary moulding material, the mould material mixture should be processed as promptly as possible, to avoid having to compensate for loss of quality of the mould material mixture due to premature curing, which in turn may result in a loss of strength in the casting moulds produced from the mould material mixture. When phenolic no-bake binders are used, the flowability of the mould material mixture is usually poor. The mould material mixture must therefore be compacted very thoroughly when producing the casting mould in order to obtain casting moulds that are as strong as possible.
The mould material mixture should be produced and processed at temperatures in the range from 15 to 35° C. If the temperature is too low, the mould material mixture is difficult to process because of the high viscosity of the phenolic no-bake resin. At temperatures above 35° C., the processing time is shortened due to premature curing of the binder.
After the casting, mould material mixtures based on phenolic no-bake binders are also able to be reprocessed, and in this case too mechanical or thermal or combined mechanical and thermal methods may be used.
As was explained previously, the acid that is used as the catalyst in furan and phenolic no-bake methods has a significant effect on the properties of the casting mould. The acid must be strong enough to ensure an adequate reaction rate while the casting mould is curing. The curing process must be easily controllable, so that sufficiently long processing times may be set. This is particularly important when producing casting moulds for very large cast items whose construction takes a relatively long period of time.
In addition, the acid must not become concentrated in the recovered substance when use sands are recovered. If acid is introduced into the mould material mixture via the recovered substances, it shortens the processing time and impairs the strength of the casting mould that is manufactured from the recovered material.
Accordingly, only a small number of acids are suitable for use as catalysts in no-bake methods. If one also takes into account financial considerations, the only acids that are viable for practical purposes are the aromatic sulfonic acids, of which toluenesulfonic acid, xylenesulfonic acid and benzenesulfonic acid are particularly important.
Phosphoric, acid and sulphuric acid are of secondary importance. As was explained previously, phosphoric acid is only suitable for curing certain furan resin qualities. However, phosphoric acid is not at all suitable for curing phenolic resins. A further disadvantage of phosphoric acid is its tendency to accumulate in the recovered material, making it more difficult to use the recovered material again. Using sulphuric acid leads to the emission of sulphur dioxide during both casting and thermal regeneration, a substance that is corrosive, hazardous to health, and foul-smelling.
During casting, the cured binder is designed to break down so that the casting mould loses its stability. The aromatic sulfonic acids used as the catalyst, particularly p-toluenesulfonic acid, benzenesulfonic acid and xylenesulfonic acid, break down under the effects of the heat and the reducing atmosphere created during casting, releasing aromatic pollutants such as benzene, toluene or xylene (BTX) besides sulphur dioxide. A fraction of these byproducts of decay also remains in the used sand and can be released during reprocessing.
Patent No. WO 97/31732 describes a self-curing furan no-bake mould material mixture for producing casting moulds that, in addition to a resin containing furan, contains methane sulfonic acid as the catalytic acid. Methane sulfonic acid may also be used in a mixture with an organic sulfonic acid or an inorganic acid. Examples of organic sulfonic acids include p-toluenesulfonic acid, benzenesulfonic acid and xylenesulfonic acid. An example of an inorganic acid would be sulphuric acid. Methane sulfonic acid has greater acidic strength than p-toluenesulfonic acid, for example. When this acid is used, the furan no-bake binder is then cured correspondingly more quickly, and curing may be achieved within acceptable periods even at low temperatures, that is to say at temperatures below 25° C. However, the use of methane sulfonic acid is associated with considerable difficulties, particularly for producing very large casting moulds, due to its strong reactivity, because it functions as a rapid curing agent, and thus only allows relatively short processing periods. Another disadvantage consists in that the use of methane sulfonic acid or methane sulfonic acid mixed with organic sulfonic acids results in the release of sulphur dioxide during casting.
Particularly because of their carcinogenic effect, extremely low MWC values (MWC=maximum workplace concentration) are imposed on hazardous aromatic substances. The MWC value for benzene is just 3.2 mg/m3, the values for toluene and xylene are 190 mg/m3 and 440 mg/mm3 respectively. This has now become a problem in foundries because highly sophisticated extraction plants and filters are needed to ensure compliance with these limit values.